Method of manufacturing solid electrolytic capacitor

A columnar pellet obtained by sintering a fine tantalum powder, constituting an anode 1, and having a length of 1 mm, a diameter of 1 mm, and a CV value (the product of an electrostatic capacitance (μF) per gram and an anodizing voltage(V)) of 23,000 μF·V/g was anodized at 60 V in an aqueous 0.1 wt % nitric acid solution to form a dielectric 2 on the surface of the pellet as shown in FIG. 2A.

This pellet was dipped in an ethanol solution containing 20 wt % of dodecylbenzene ferric sulfonate at room temperature for 1 minute, and then dipped in a methanol solution containing 50 wt % of a pyrrole monomer at room temperature for 1 minute. Thereafter, the resultant pellet was kept in the atmospheric air for 10 minutes to perform chemical oxidation and polymerization and doping at once. The above series of processes: the process of filling the oxidant, the process of bringing the pellet into contact with the pyrrole monomer, and the process of performing polymerization and doping were performed ten times, thereby forming a black polypyrrole layer on the dielectric 2 as a first conductive polymer compound layer 3, as shown in FIG. 2B.

5 g of a polyaniline powder soluble in an organic solvent prepared by the conventional known method described in Reference 1 or the like were gradually added to 95 g of N-methyl-2-pyrrolidone to dissolve the polyaniline powder, thereby obtaining a blue-black solution. The pellet was dipped in this solution at room temperature for 1 minute, and then kept in an atmosphere at 100° C. to form an undoped polyaniline film. The resultant structure was dipped in an aqueous sulfuric acid solution for 30 minutes to perform doping, thereby forming a second conductive polymer compound layer 4 on the first conductive polymer compound layer 3, as shown in FIG. 2C.

In addition, a silver paste layer serving as a graphite layer 5 and a cathode electrode 6 was formed on the second conductive polymer compound layer 4, and an anode lead 7 and a cathode lead 8 extended. Thereafter, the resultant structure was sealed with an epoxy resin constituting a molding resin 13, thereby completing a solid electrolytic capacitor shown in FIG. 1A and having the sectional structure of FIG. 1B.

The capacitance reproduction ratios, impedances at 100 kHz, and leakage currents, of the completed solid electrolytic capacitor, which are obtained before and after a solder heat test (dipped at 260° C. for 10 seconds) are shown in Table 1.

As shown in FIG. 2A, a dielectric 2 was formed by the same method as described in Example 1 using a pellet identical to that in Example 1.

This pellet was dipped in an aqueous 30 wt % potassium dichromate solution of at room temperature for 1 minute, and then dipped in an aqueous 5 wt % aniline monomer solution containing an aniline monomer and p-toluenesulfonic acid at an equi-molar ratio at room temperature for 1 minute. Thereafter, the resultant structure was kept in the atmospheric air for 30 minutes to perform chemical oxidation and polymerization and doping to the resultant structure. The series of processes: the process of filling the oxidant, the process of bringing the pellet into contact with the pyrrole monomer, and the process of performing polymerization and doping were performed five times, as shown in FIG. 2B, to form a black polyaniline layer on the dielectric 2 as a first conductive polymer compound layer 3.

As in Example 1, 5 g of a polyaniline powder were gradually added to N-methyl-2-pyrrolidone to dissolve the polyaniline powder, thereby obtaining a blue-black solution. The pellet was dipped in this solution at room temperature for 1 minute, and then kept in an atmosphere at 100° C. to form an undoped polyaniline film. Thereafter, the resultant structure was dipped in p-toluenesulfonic acid for 30 minutes to perform doping, thereby forming a second conductive polymer compound layer 4 on the first conductive polymer compound layer 3, as shown in FIG. 2C.

In addition, a silver paste layer serving as a graphite layer 5 and a cathode electrode 6 was formed on the second conductive polymer compound layer 4, and an anode lead 7 and a cathode lead 8 extended. Thereafter, the resultant structure was sealed with an epoxy resin, thereby completing a solid electrolytic capacitor shown in FIG. 1A and having the sectional structure of FIG. 1B.

The capacitance reproduction ratios, impedances at 100 kHz, and leakage currents, of the completed solid electrolytic capacitor, which are obtained before and after a solder heat test (dipped at 260° C. for 10 seconds) are shown in Table 1.

Comparative Example 1

FIG. 3 shows the sectional structure of a solid electrolytic capacitor manufactured by a conventional manufacturing method. This method will be described below with reference to FIG. 3.

A dielectric 32 was formed by the same method as described in Example 1 using a pellet identical to that in Example 1.

The pellet constituting an anode 31 was dipped in an ethanol solution containing 20 wt % of dodecylbenzene ferric sulfonate at room temperature for 1 minute, and then dipped in a methanol solution containing 50 wt % of a pyrrole monomer at room temperature for 1 minute. Thereafter, the pellet was kept in the atmospheric air for 10 minutes to perform chemical oxidation and polymerization and doping at once. The above series of processes: the process of filling the oxidant, the process of bringing the pellet into contact with the pyrrole monomer, and the process of performing polymerization and doping were performed ten times to form a black polypyrrole layer on the dielectric 32 as a conductive polymer compound layer 33.

A silver paste layer serving as a graphite layer 35 and a cathode electrode 36 was formed on the conductive polymer compound layer 33, and an anode lead and a cathode lead extended. Thereafter, the resultant structure was sealed with an epoxy resin 37, thereby completing a solid electrolytic capacitor.

The capacitance reproduction ratios, impedances at 100 kHz, and leakage currents, of the completed solid electrolytic capacitor, which are obtained before and after a solder heat test (dipped at 260° C. for 10 seconds) are shown in Table 1. Comparative Example 2

FIG. 4 shows the sectional structure of a solid electrolytic capacitor manufactured by another conventional manufacturing method. The solid electrolytic capacitor will be described below with reference to FIG. 4.

A dielectric 42 was formed by the same method as described in Example 1 using a pellet identical to that in Example 1.

5 g of a polyaniline powder soluble in an organic solvent prepared by the conventional known method described in Reference 1 or the like were gradually added to 95 g of N-methyl-2-pyrrolidone to dissolve the polyaniline powder, thereby obtaining a blue-black solution. The pellet constituting an anode 41 was dipped in this solution at room temperature for 1 minute, and then kept in an atmosphere at 100° C. to form an undoped polyaniline film. Therefore, the resultant structure was dipped in an aqueous sulfuric acid solution for 30 minutes to perform doping and to make the undoped polyaniline film conductive, thereby forming a conductive polymer compound layer 44 on the dielectric 42.

A silver paste layer serving as a graphite layer 45 and a cathode electrode 46 was formed on the conductive polymer compound layer 44, and an anode lead and a cathode lead extended. Thereafter, the resultant structure was sealed with an epoxy resin 47, thereby completing a solid electrolytic capacitor.

The capacitance reproduction ratios, impedances at 100 kHz, and leakage currents, of the completed solid electrolytic capacitor, which are obtained before and after a solder heat test (dipped at 260° C. for 10 seconds) are shown in Table 1.

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Capacitance Reproduction Ratio, High-Frequency

Impedance, and Leakage Current

Item

Capacitance

Reproduction

Leakage

Ratio Current Impedance

Sample C/Co (%) LC (nA) R (mΩ)

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Before 93 40 450

Test

After 91 75 480

Test

Before 91 45 475

Test

After 89 83 483

Test

Compara-

Before 93 37 445

tive Test

After 85 5,000 565

Test

Compara-

Before 75 43 650

tive Test

After 73 92 685

Test

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Impedance R; measurement frequency f = 100 kHz

Test; solder heat test (dipped at 260° C. for 10 seconds)

As shown in Table 1, in each of Examples 1 and 2 of the present invention, excellent results are obtained. That is, each solid electrolytic capacitor has a high capacitance reproduction ratio and excellent impedance characteristics in a high-frequency range, and is free from an increase in leakage current caused by thermal stress generated when the solid electrolytic capacitor is mounted.

In Examples 1 and 2 described above, when the first conductive polymer compound layer 3 is to be formed on the dielectric oxide coating 2, the chemical oxidation and polymerization process and the doping process are performed at once. However, as in Example 3 (will be described next), these two processes can be separately performed as follows. That is, an undoped polymer compound layer is formed, and this layer is doped with a dopant to be conductive, thereby forming a first conductive polymer compound layer 3. Processes performed until the first conductive polymer compound layer 3 is formed will be described below.

In Example 3, a dielectric 2 is formed by the same method as described in Example 1 using a pellet identical to that in Example 1. Thereafter, the pellet constituting an anode 1 is dipped in an aqueous 20 wt % ammonium peroxodisulfate solution at room temperature for 1 minute, and then dipped in an aqueous 5 wt % aniline monomer solution at room temperature for 1 minute, thereby forming an undoped polyaniline film on the dielectric 2. The resultant structure is dipped in p-toluenesulfonic acid for 30 minutes to perform doping, thereby making the polyaniline film conductive. The above series of processes (the process of filling an oxidant, the process of bringing the pellet into contact with the aniline monomer, and the process of performing doping) are performed five times, thereby forming a conductive polyaniline film serving as a first conductive polymer compound layer 3.

As described above, in a method of manufacturing a solid electrolytic capacitor according to the present invention, the second conductive polymer compound layer 4 must be formed such that an undoped polymer compound layer is formed and then made conductive. However, the first conductive polymer compound layer 3 may be formed such that the chemical oxidation and polymerization process and the doping process are performed at once or separately.

As has been described above, according to the present invention, a conductive polymer compound layer obtained by performing a chemical oxidation and polymerization using an oxidant on a dielectric oxide coating is formed as a first conductive polymer compound layer, and a conductive polymer compound layer which is made conductive by doping proton acid in a polymer compound which is polymerized in advance and soluble in an organic solvent is formed as a second conductive polymer compound layer. Therefore, there can be provided a method of manufacturing a solid electrolytic capacitor which constitutes a solid electrolyte having a two-layered structure obtained by combining the first conductive polymer compound layer to the second conductive polymer compound layer, has a high capacitance reproduction ratio and excellent frequency characteristics, and is free from an increase in leakage current caused by thermal stress generated when the solid electrolytic capacitor is mounted.

In this case, as the first conductive polymer compound layer, one of polypyrrole and polyaniline obtained by performing chemical oxidation and polymerization using an oxidant is preferably used due to the following two reasons. First, a polymer compound which can uniformly and tightly adhere to the entire fine uneven surface of a dielectric oxide coating is preferably used. A solid electrolytic capacitor having a high capacitance reproduction ratio can be obtained by causing the polymer compound to uniformly and tightly adhere to the entire uneven surface of the dielectric oxide coating. Second, a conductive polymer compound having a high conductivity is preferably used.

A reason why a polymer compound which is polymerized in advance, soluble in an organic solvent, and made conductive by doping proton acid in the polymer compound is used as the material of the second conductive polymer compound layer is that a thick conductive polymer compound film can be easily formed. When the thick conductive polymer compound film is formed, the above problem of an increase in leakage current generated when the solid electrolytic capacitor is mounted can be solved.

When the two-layered structure is formed by combining the first conductive polymer compound layer to the second conductive polymer compound layer, the drawbacks of the two layers are compensated for each other. Therefore, a method of manufacturing a solid electrolytic capacitor which has a high capacitance reproduction ratio and excellent frequency characteristics and is free from an increase in leakage current caused by thermal stress generated when the solid electrolytic capacitor is mounted can be provided.